I. Field of the Invention
This invention relates generally to implantable monitoring devices and implantable cardiac therapy devices, and more particularly to a method for monitoring the respiratory patterns of a chronic heart failure patient to track changes in disease state.
II. Description of the Related Art
Congestive heart failure (CHF) is a debilitating, end-stage disease in which abnormal function of the heart leads to inadequate blood flow to fulfill the needs of the body""s tissues. Typically, the heart loses propulsive power because the cardiac muscle loses capacity to stretch and contract. Often, the ventricles do not adequately fill with blood between heartbeats and the valves regulating blood flow may become leaky, allowing regurgitation or backflow of blood. The impairment of arterial circulation deprives vital organs of oxygen and nutrients. Fatigue, weakness, and inability to carry out daily tasks may result.
Not all CHF patients suffer debilitating symptoms immediately. Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive.
As CHF progresses, it tends to become increasingly difficult to manage. Even the compensatory responses it triggers in the body may themselves eventually complicate the clinical prognosis. For example, when the heart attempts to compensate for reduced cardiac output, it adds muscle causing the ventricles to grow in volume in an attempt to pump more blood with each heartbeat. This places a still higher demand on the heart""s oxygen supply. If the oxygen supply falls short of the growing demand, as it often does, further injury to the heart may result. The additional muscle mass may also stiffen the heart walls to hamper rather than assist in providing cardiac output.
CHF has been classified by the New York Heart Association (NYHA). Their classification of CHF corresponds to four stages of progressively worsening symptoms and exercise capacity from Class I to Class IV. Class I corresponds to no limitation wherein ordinary physical activity does not cause undue fatigue, shortness of breath, or palpitation. Class II corresponds to slight limitation of physical activity wherein such patients are comfortable at rest, but where ordinary physical activity results in fatigue, shortness of breath, palpitations, or angina. Class III corresponds to a marked limitation of physical activity wherein, although patients are comfortable at rest, less than ordinary activity will lead to symptoms. Lastly, Class IV corresponds to inability to carry on any physical activity without discomfort, wherein symptoms of CHF are present even at rest and where with any physical activity, increased discomfort is experienced.
Current standard treatment for heart failure is typically centered around medical treatment using ACE inhibitors, diuretics, and digitalis. It has also been demonstrated that aerobic exercise may improve exercise tolerance, improve quality of life, and decrease symptoms. Only an option in 1 out of 200 cases, heart transplantation is also available. Other cardiac surgery is also indicated for only a small percentage of patients with particular etiologies. Although advances in pharmacological therapy have significantly improved the survival rate and quality of life of patients, patients in NYHA Classes III or IV, who are still refractory to drug therapy, have a poor prognosis and limited exercise tolerance. Cardiac pacing has been proposed as a new primary treatment for patients with drug-refractory CHF.
Heart failure patients require close medical management to reduce morbidity and mortality. Because the disease status evolves with time, frequent physician follow-up examinations are often necessary. At follow-up, the physician may make adjustments to the drug regimen in order to optimize therapy. This conventional approach of periodic follow-up may be less satisfactory for heart failure, in which acute, life-threatening exacerbations can develop between physician follow-up examinations. It is well known among clinicians that if a developing exacerbation is recognized early, it can be more easily and inexpensively terminated, typically with a modest increase in oral diuretic. However, if it develops beyond the initial phase, an acute heart failure exacerbation becomes difficult to control and terminate. Hospitalization in an intensive care unit is often required. It is during an acute exacerbation of heart failure that many patients succumb to the disease. Early identification may also allow for pacing therapy from an implanted pulse generator.
In patients with CHF, alterations of breathing patterns such as Cheyne-Stokes respiration or periodic breathing have been identified during both day- and night-time. Periodic breathing is defined as a waxing and waning of tidal volume without periodic phases of apnea, whereas Cheyne-Stokes respiration is defined as a type of periodic breathing that includes periods of apnea between the phases of hyperventilation (hyperpnea) and hypoventilation (hypopnea.) These alterations of breathing are associated with marked oscillations of arterial oxygen saturation and heart rate.
Detection of periodicity and the underlying frequency content of a signal such as arterial oxygen saturation, breath to breath interval, tidal volume amplitude or heart rate (R to R intervals) can be accomplished via any of several widely used techniques of power spectral analysis. These techniques include, but are not limited to, periodogram estimation, the maximum entropy (all poles) method and the Blackman-Tukey algorithm.
Conventional cardiac monitors, such as defibrillators, pacemakers, Holter monitors, and cardiac event records, are tailored for the diagnosis and/or therapy of abnormalities of the cardiac electrical system. In contrast, heart failure is a disease of the cardiac mechanical system: it is primarily a failure of the myocardium to meet the mechanical pumping demands required of it. In monitoring the status of a heart failure patient, measuring the mechanical hemodynamic variables is clearly desirable. Examples of mechanical hemodynamic variables include atrial, ventricular, and arterial pressures, and cardiac output (volume of blood pumped into the aorta per unit time). However, because of the complex feedback network that monitors and controls cardiac performance, measuring variables that do not directly reflect the mechanical performance of the heart is also useful.
Some limitations of monitoring systems have been addressed by the development of an implantable system that monitors hemodynamic status (Medtronic Chronicle, Medtronic, Inc., Minneapolis, Minn.). While this system potentially avoids the need for active patient participation in disease state monitoring, it relies on an intravascular pressure transducer placed in the right ventricle of the heart. Examples of other hemodynamic monitoring systems include U.S. Pat. No. 5,454,838 in which Vallana et al. teach placement of a sensor on the myocardial wall using an intravascular approach. In U.S. Pat. No. 5,496,351, Plicchi et al. propose placing a sensor within the myocardial wall. Mortazavi in U.S. Pat. No. 5,040,538 and Cohen et al. in U.S. Pat. No. 4,815,469 describe placement of an optical sensor within the right ventricle.
An Implantable Ambulatory Electrocardiogram Monitor is described by Nappholz et al. in U.S. Pat. No. 5,113,869, incorporated herein by reference. This device is designed for chronic extravascular implantation. In contrast to cardiac recorders, it performs analysis on the electrocardiogram signal in order to predict imminent cardiac arrhythmias and to detect cardiac ischemia. Like the cardiac recorders, it is capable of storing raw ECG data for later review by a physician. This feature, along with the record of arrhythmic events it detected, allows the physician to tailor pharmacologic therapy. In addition, Nappholz et al. mention the use of transthoracic impedance for minute ventilation, ultrasound transducers for arterial pressure, or other sensors to allow discrimination of arrhythmias from normal cardiac rhythms caused by exertion or physiologic stress.
The use of impedance plethysmography, also called minute ventilation or minute volume, to control an exercise responsive or rate adaptive pacemaker is well know in the art. See for example U.S. Pat. No. 4,702,253 to Nappholz et al., U.S. Pat. No. 5,197,467 to Steinhaus et al. and U.S. Pat. No. 5,562,711 to Yerich et al., which patents are incorporated herein by reference. Minute ventilation is defined by the equation:
VE=RRxc3x97TV
where RR=respiration rate in breaths per minute, and TV=tidal volume in liters.
Clinically, the measurement of VE is performed by having the patient breathe directly into a device that measures the exchange of air and computing the total volume per minute. The direct measurement of VE is not practical with an implanted device. However, measurement of the impedance changes of the thoracic cavity can be implemented with an implanted pacemaker, and transthoracic impedance has been shown to correlate well with VE. A pacemaker that is provided with impedance measurement capabilities is disclosed in the Nappholz et al. ""253 patent mentioned above. The magnitude of the change of the impedance signal corresponds to the tidal volume and the frequency of change corresponds to respiration rate. Thus, measurement of thoracic impedance can be used as one method for obtaining both respiratory rate and tidal volume data.
In practice, thoracic impedance can be measured through assessment of the impedance present between two or more cardiac electrodes, such as the electrodes otherwise used for pacing and/or sensing in connection with a cardiac pacemaker. In particular, it has been shown that thoracic impedance can be measured by delivering constant-current excitation pulses between two xe2x80x9csourcexe2x80x9d electrodes, such that the current is conducted through some region of the thoracic cavity. The voltage differential between two xe2x80x9crecordingxe2x80x9d electrodes can then be measured to ascertain the impedance as reflected by the voltage differential arising from the conduction of the excitation current pulses through the thorax.
Because of the considerations described above, the principal object of the present invention is to provide a method for use of a device that monitors a patient""s hemodynamic status.
Another object of the invention is to monitor the status of a patient with chronic heart failure in order to optimize patient therapy.
A further object is to monitor the status of a chronic heart failure patient in order to recognize and facilitate the early termination of a developing exacerbation.
Further objects and advantages will become apparent from a consideration of the ensuing description and drawings
The present invention provides a method for monitoring changes in the hemodynamic status of a heart failure patient by monitoring respiratory patterns. An implantable or other ambulatory monitor senses one or more of several physiologic signals to identify breathing patterns. The physiologic signals include transthoracic impedance, intracardiac electrograms and blood oxygen content. These physiologic signals are processed to generate physiologic parameter data that include the patient""s respiratory rate, respiratory tidal volume, heart rate or arterial oxygen saturation. The physiologic parameter data are analyzed to derive an indication of periodic breathing. This is accomplished in a first preferred embodiment by a thresholding/binning technique to determine cycles of hyperpnic and hypopnic breathing within a periodic breathing episode. In a second preferred embodiment of the invention, this is accomplished by power spectral analysis of the physiologic parameter data.
The physiologic parameter data are derived from the physiologic signal that is sensed using a known physiologic sensor. In a preferred embodiment for the physiologic data of respiratory rate and tidal volume, transthoracic impedance is used providing a signal that changes with the lungs filling with air and expelling air.
Some physiologic parameter data such as respiratory rate, respiratory tidal volume, and heart rate are not continuous measures, but rather are defined at discrete intervals as suggested by the underlying process. For example, respiratory tidal volume and respiratory rate are both derived on a per-breath basis; thus, successive samples of these data would preferably be obtained at breath-to-breath intervals. As another example, heart rate is derived on a per-heartbeat basis; thus, successive samples of these data would preferably be obtained at heartbeat-to-heartbeat intervals. In contrast, other physiologic parameter data do not have an inherently preferred sampling interval. One such example is arterial oxygen saturation. For these cases, any convenient sampling interval could be adopted, for example at an interval related to heartbeat intervals or breath intervals. Determining the presence of periodic breathing provides an indicator of a current or impending exacerbation of a chronic heart failure condition. Appropriate steps such as initiation of pacing therapy to the heart or providing a warning to the patient or a healthcare provider of the condition may then be taken.
In one example of a preferred embodiment, respiratory tidal volume is monitored and a flag is set when the tidal volume exceeds or falls below predetermined thresholds indicative of hyperpnea or hypopnea and thus outside the normal breathing range. The time difference between repeated settings of this flag are tracked to determine if the difference is regular. Such a regular pattern is an indicator of periodic breathing which is itself an indicator of a CHF exacerbation. It is preferred that this monitoring occur during the time when the patient is at rest or asleep to avoid the introduction of extraneous influences such as may be present with patient exercise or other activity. Additionally, a long cycle length, on the order of about 15 seconds, or at least 3 breaths for excursions above or below the thresholds may be used as an added requirement to provide a clear indication of hyperpnea or hypopnea, respectively.
In an alternative embodiment of the invention, respiratory rate is monitored. The duration of excursions above a first threshold and below a second threshold are used as indicators of hyperpnea and hypopnea. As with the embodiment described above, these excursions provide an indicator of periodic breathing and a potential CHF exacerbation. Alternatively, power spectral analysis can be performed on respiratory rate or respiratory tidal volume data.
In another alternative embodiment of the invention, an electrogram signal is sensed from a sensing electrode. Heartbeat R-R intervals are calculated and a power spectrum analysis is performed on a sequence of such intervals. Changes in the very low frequency component of a power spectrum of R-R intervals are used to indicate the presence of periodic breathing. An analysis of heart rate excursions with the binning/thresholding method can also be used to indicate periodic breathing.
In yet another alternative embodiment of the invention, a blood oxygen saturation sensor is provided. Periodic changes in blood oxygen saturation are monitored as the indicator of periodic breathing. These data may be analyzed using either power spectral analysis or looking for excursions above and below predetermined thresholds.